Composite backed prestressed mirror for solar facet

ABSTRACT

A glass structure, such as a mirror facet, having a glass member, a composite structure and a support structure. The composite structure includes a rigid interlayer which is bonded to the glass member and exerts a compressive force thereon to place the glass member in compression. The support structure is used to mount the glass structure and prevents the glass member from collapsing due to the compressive force exerted by the rigid interlayer. The glass structure is particularly well adapted for use in forming heliostats, parabolic dishes, trough concentrators, or other like elements for use in solar power systems, and does not suffer from the limitations or prior forms of such devices.

TECHNICAL FIELD

[0001] The present invention relates generally to the construction ofmirrors and more particularly relates to a prestressed mirror and amethod for fabricating the same.

BACKGROUND OF THE INVENTION

[0002] 1. Background Art

[0003] High concentration solar thermal power systems typically rely ona field of heliostats, or a parabolic dish or trough concentrators totrack the sun and reflect solar radiation to a receiver where the solarenergy heats a working fluid, such as steam. The working fluid is thenemployed to provide thermal energy for various industrial and commercialprocesses or to produce electricity. Similarly, concentratingphotovoltaic systems use mirrors of varying types to collect solarenergy where it is turned directly into electrical energy.

[0004] In such systems, it is critical for performance objectives thatthe mirror facets which make up these systems meet stringent opticalperformance characteristics such as radius of curvature, reflectivityand surface slope error. It is also critical that these mirror facets belightweight so as to reduce the cost associated with the drive unitsthat are needed to aim the mirror facets. The mirror facets must also besufficiently robust to ensure a long life despite their exposure toprecipitation, wind and sun. Consequently, these mirror facets must becapable of withstanding sustained winds in excess of 100 m.p.h.,temperatures ranging from −40° F. to 130° F., impacts from hail,corrosive elements (e.g., acid rain, salt), humidity changes, etc.Furthermore, as there may be hundreds or even thousands of mirror facetsin a system, it is highly desirable that the mirror facet be of highlycost efficient construction.

[0005] The designs of conventional mirror facets have relied on thethickness of the glass that forms the mirror facet and/or the framestructure of the mirror facet to compensate for the relatively weaktensile properties of glass. This approach has several drawbacks,including losses in reflectivity as a result of the use of relativelythicker glass and a relatively higher weight. Additionally, these mirrorfacets are not as robust as desired, being highly susceptible to damageduring shipping, installation and use. Furthermore, as these mirrorfacets have relatively weak tensile properties, their exposure totime-varying forces such as wind can cause the propagation of crackswhich could permit the reflective finish of the mirror facet to corrode,with the result being impaired performance of the mirror facet.

[0006] In view of these drawbacks, some conventional mirror facets haveobtained additional strength through the use of operations such asslumping, chemical strengthening, annealing and/or tempering. Theseprocesses tend to be relatively expensive, and as such, a substantialcost penalty is incurred if these processes are employed. Furthermore,these mirror facets typically rely on relatively thicker glass and assuch are accompanied by drawbacks such as losses in reflectivity andhigher weight.

SUMMARY OF THE INVENTION

[0007] It is one object of the present invention to provide a glassstructure which is robust yet light in weight and relatively inexpensiveto manufacture.

[0008] It is another object of the present invention to provide a glassstructure which is robust yet utilizes a relatively thin glass member.

[0009] It is yet another object of the present invention to provide aglass structure which employs a structure that applies a compressiveforce to a glass member to place the glass member in compression as toimprove the strength of the glass structure.

[0010] It is a further object of the present invention to provide aglass structure which employs a relatively lightweight reinforcingmember that does not affect the surface slope error of the glassstructure.

[0011] It is yet another object of the present invention to provide amethod for forming a glass structure.

[0012] In one preferred form, the present invention provides a glassstructure having a glass member, a composite structure and a supportstructure. The composite structure includes a rigid interlayer which isbonded to the glass member and exerts a compressive force thereon toplace the glass member in compression. The support structure is used tomount the glass structure and prevents the glass member from collapsingdue to the compressive force exerted by the rigid interlayer.

[0013] In another preferred form, the present invention provides amethod for forming a glass structure comprising the steps of providing aglass member and securing a rigid interlayer to the glass member suchthat the rigid interlayer applies a compressive force to the glassmember.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Additional advantages and features of the present invention willbecome apparent from the subsequent description and the appended claims,taken in conjunction with the accompanying drawings, wherein:

[0015]FIG. 1 is a schematic illustration of a heliostat-type solar powersystem having a plurality of glass structures each constructed inaccordance with a preferred embodiment of the present invention;

[0016]FIG. 2 is a perspective view of a portion of the glass structureof FIG. 1;

[0017]FIG. 3 is an enlarged perspective view of the glass structure ofFIG. 1;

[0018]FIG. 4 is a schematic view of the glass structure of FIG. 1 beingfabricated on a vacuum tool;

[0019]FIG. 5 is a schematic illustration similar to that of FIG. 1 butshowing a parabolic dish configuration; and

[0020]FIG. 6 is a schematic illustration similar to that of FIG. 1 butshowing a parabolic trough configuration.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0021] With reference to FIG. 1 of the drawings, an illustrative solarpower system is generally indicated by reference numeral 10. Solar powersystem 10 is shown to include an elevated receiver 12 and a plurality ofheliostats 14. Each of the heliostats 14 has a base structure 16 and adrive mechanism 18, as well as a glass structure 20 that is constructedaccording to a preferred embodiment of the present in invention. Basestructure 16 and drive mechanism 18 are conventional in theirconstruction and operation and as such, need not be discussed in detail.Briefly, base structure 16 supports drive mechanism 18 and glassstructure 20. Drive mechanism 18 selectively orients glass structure 20in a predetermined manner such that incident rays of solar energy 24 arereflected to receiver 12. Accordingly, drive mechanism 18 operates tochange the position (e.g., angularity) of glass structure 20 to trackthe position of the sun.

[0022] With additional reference to FIGS. 2 and 3, glass structure 20 isshown to include a glass member 30, a composite structure 32 and asupport structure 34. Glass member 30 preferably includes a relativelythin glass panel 40 having a thickness of about 0.001 inches to about0.4 inches. The rear side 42 of glass panel 40 is coated with areflective material 44, such as silver, a topcoat 46, such as copper,and mirror backing paint.

[0023] Composite structure 32 includes a rigid interlayer 50 that isbonded to and applies a compressive force to glass member 30. Rigidinterlayer 50 may be formed from a resin such as an unsaturatedpolyester, a bismaleimide (BMI), an epoxy vinyl ester or another epoxy,which is applied to the rear side of glass member 30 while in a liquidstate by any practical means, including brushing, curtain coating,spraying and/or extrusion. The resin is then cured to increase thethickness of the assembly (i.e., the glass member 30 and the rigidinterlayer 50) to provide increased durability. It should be noted thatdeformation of the assembly due to stress loads is reduced as thethickness of the assembly is increased, with the deformation beingapproximately inversely proportional to the cube of the thickness of theassembly. Thus, incorporation of rigid interlayer 50 into glassstructure 20 limits the deformation of glass structure 20 even where arelatively thin glass member 30 is used. This permits the thickness ofglass member 30 to be reduced so as to improve the reflectivity of glassstructure 20. Besides increasing the thickness and relative stiffness ofglass structure 20, rigid interlayer 50 also applies a compressive forceto glass member 30. In the particular embodiment illustrated, the resinforming rigid interlayer 50 shrinks as it cures, thus applying acompressive force to glass member 30. Inorganic filler materials, suchas calcium carbonate, may be incorporated into the liquid resin tocontrol the shrinkage of the rigid interlayer while it is being cured.

[0024] In the example provided, composite structure 32 is also shown toinclude a reinforcing member 60 which further increases the thickness ofglass structure 20 and its resistance to deformation. In the particularembodiment shown, reinforcing member 60 is a polymeric matrix compositecontaining a woven fiberglass mat reinforcement. The fiberglass matreinforcement is initially saturated in liquid resin and subsequentlyplaced onto rigid interlayer 50. The resin in preferably the same resinused to form rigid interlayer 50 (i.e., an unsaturated polyester, abismaleimide (BMI), an epoxy vinyl ester or another epoxy).

[0025] Support structure 34 is placed onto the fiberglass matting whilethe resin is still wet. Support structure 34 is adapted for use inmounting glass structure 20 to base structure 16 and spreading loadstransmitted between glass structure 20 and base structure 16 over arelatively large area. Support structure 34 may be made from anystructural material in any appropriate structural shape. Supportstructure 34 is preferably fabricated from a high-strength, low-cost andlow-weight material such as fiberglass. Alternatively, support structure34 may be fabricated from a metallic material such as steel or aluminum.In the particular embodiment illustrated, support structure 34 includesa plurality of hat-shaped beam sections 70, with each section 70 beingformed from a continuous strip to include a pair of flanges 72, a pairof upwardly directed wall members 74 and a generally flat mount 76 asshown best in FIG. 3. Curing of the resin in reinforcing member 60 bondsthe flanges of the support structure 34 to composite structure 32 andcauses reinforcing member 60 to apply an additional compressive force toglass member 30.

[0026] In FIG. 4, a tool for fabricating glass structure 20 is generallyindicated by reference numeral 80. Tool 80 is shown to have a contouredsurface 82 through which a plurality of feed holes 84 have been drilled.The feed holes 84 terminate at a central manifold 86, which is coupledto a pressure gauge 88 and a shut-off valve 90. Glass member 30 isinitially placed on the contoured surface 82 of tool 80 such that thetransparent surface of glass member 30 is in contact with the contouredsurface 82. A vacuum is applied through valve 90 to the central manifold86, causing glass member 30 to sealingly contact the contoured surface82. This places the front surface of the glass member 30 in compressionand the rear surface in tension. Vacuum pressure is maintained throughthe valve 90 by a conventional vacuum source, such as a vacuum pump, toensure maintenance of the desired contour during the entire fabricationprocess. Those skilled in the art will understand that the magnitude ofthe vacuum may be maintained at a predetermined level throughout thefabrication process or may be varied, depending on a number of factorsthat are particular to a specific application and need not be detailedherein. Vacuum gauge 88 is used to ensure the proper vacuum level ismaintained.

[0027] Contoured surface 82 is fabricated to a predetermined shape thattakes into account the compressive forces that are developed through thecuring of resin, as well as the spring-like nature of the components ofthe glass structure 20 which cause the glass structure 20 to relaxsomewhat after it is removed from the tool. Resin which forms rigidinterlayer 50 is next applied to the rear surface of glass member 30 andcured. As mentioned above, the rear surface of glass member 30 isinitially in tension. However, as the resin shrinks when it cures, itgenerates a compressive force which is applied to the rear surface ofglass member 30. The compressive force is of sufficient magnitude toplace all of glass member 30 (i.e., both the front and rear surfaces) incompression. It should be noted that the resin is preferably cured at atemperature that is greater than or equal to the maximum operatingtemperature of the glass structure 20 (i.e., the curing temperatureshould meet or exceed the maximum temperature that the glass structure20 will be exposed to during its operation) so as to prevent the resinfrom permanently changing dimensionally during the use of the glassstructure 20 or decreasing the desired compressive force by expansion ofthe resin which forms rigid interlayer 50 relative to glass member 30 byshrinkage of the resin which forms rigid interlayer 50 and shrinkage ofthe composite material 60 relative to the glass member 30.

[0028] After rigid interlayer 50 has cured, reinforcing member 60 isapplied to rigid interlayer 50. Support structure 34 is then positionedonto reinforcing member 60 such that the flanges 72 contact the resin.The liquid resin is then cured at an elevated temperature as discussedin the immediately preceding paragraph which details the formation ofthe rigid interlayer 50. Support structure 34 is bonded to reinforcingmember 60 as the resin forming reinforcing member 60 cures to therebyprovide structural support for glass member 30. Once the curing of thereinforcing member 60 is complete, the vacuum in central manifold 86 isreleased to permit glass structure 20 to be removed from tool 80. Asmentioned above, support structure 34 provides structural support toglass structure 20 and prevents the residual compressive forces fromcollapsing the glass member 30.

[0029] It is important to note that rigid interlayer 50 provides auniform and continuous surface for the mounting of glass member 30. Incontrast, if glass member 30 were to be mounted directly to reinforcingmember 60, the small voids between the reinforcing fibers would leavethe glass member 30 unsupported in the area of the void, therebypermitting the glass member 30 to dimple in response to the compressiveforces that are developed when the resin cures. Accordingly, in a glassstructure constructed in this manner (i.e. without rigid interlayer 50),the surface of the glass structure obtains an orange peel-like texturewhich tends to increase the surface slope error of the glass structure,resulting in a substantial decrease in the power delivered to receiver12.

[0030] While the glass structure 20 has been described thus far as beingemployed in a heliostat, those skilled in the art will appreciate thatthe invention, in its broader aspects, may be constructed somewhatdifferently. For example, the glass structure may form a single facet ofa relatively large heliostat, or a single facet of a parabolic dishconcentrator (FIG. 5) or a parabolic trough concentrator (FIG. 6).Accordingly, while the invention has been described in the specificationand illustrated in the drawings with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention asdefined in the claims. In addition, many modifications may be made toadapt a particular situation or material to the teachings of theinvention without departing from the essential scope thereof. Therefore,it is intended that the invention not be limited to the particularembodiment illustrated by the drawings and described in thespecification as the best mode presently contemplated for carrying outthis invention, but that the invention will include any embodimentsfalling within the foregoing description and the appended claims.

What is claimed is:
 1. A method for forming a glass structurecomprising: providing a glass member with a generally planar surface;and securing a rigid interlayer to the glass member such that the rigidinterlayer applies a compressive force to the surface of the glassmember.
 2. The method of claim 1, further comprising: securing areinforcing structure to the rigid interlayer; and securing a supportmember to the reinforcing structure.
 3. The method of claim 2, whereinsecuring the reinforcing structure to the rigid interlayer includes:providing a reinforcing member; applying a resin to the reinforcingmember to form the reinforcing structure; applying the reinforcingstructure to the rigid interlayer; and curing the resin to bond thereinforcing structure to the rigid interlayer.
 4. The method of claim 3,wherein securing the reinforcing structure to the rigid interlayer andsecuring the support member to the reinforcing structure are performedsubstantially simultaneously when the resin is cured.
 5. The method ofclaim 3, wherein curing the resin is performed at a predeterminedtemperature that is greater than or equal to a maximum temperature atwhich the glass structure will be used.
 6. The method of claim 1,wherein after providing the glass member, the method includes formingthe glass member to a predetermined shape.
 7. The method of claim 6,wherein the glass member is formed on a vacuum tool.
 8. The method ofclaim 6, wherein the step of securing the rigid interlayer to the glassmember includes: applying a resin over an area corresponding to a rearsurface of the glass member; and curing the resin to form the rigidinterlayer.
 9. The method of claim 8, wherein curing the resin isperformed at a predetermined temperature that is greater than or equalto a maximum temperature at which the glass structure will be used. 10.The method of claim 1, wherein the glass member is a mirror.
 11. Amethod for forming a mirror assembly comprising: providing a mirrorhaving a front surface that is associated with light reflection and arear surface; applying a resin to the rear surface of the mirror; andcuring the resin; wherein the resin shrinks as it cures and applies acompressive force to the rear surface.
 12. The method of claim 11,wherein the compressive force has a magnitude that is sufficient todrive the whole of the mirror into a state of compression.
 13. Themethod of claim 12, wherein the mirror is preformed such that the frontsurface of the mirror conforms to a non-flat shape.
 14. The method ofclaim 13, wherein the non-flat shape is selected from at least one of:shapes formed at least in part by a spherical radius and parobalicshapes.
 15. The method of claim 11, wherein the resin is selected fromat least one of unsaturated polyesters, bismaleimides, epoxy vinylesters and epoxies.
 16. The method of claim 11, further comprising:providing a reinforcing structure; and securing the reinforcingstructure to the rear surface of the mirror, the reinforcing structuresupporting the mirror.
 17. The method of claim 16, wherein thereinforcing structure includes an interlayer and a support structure.18. The method of claim 11, wherein the mirror includes a glass panelhaving a thickness of about 0.001 inch to about 0.4 inch.
 19. A methodfor forming a mirror assembly comprising: providing a glass memberhaving a glass panel with a thickness that is less than about 0.4 inchthick and a reflective material that is associated with a surface of theglass panel to effect light reflection, the glass member having a frontsurface and a rear surface; forming the glass member such that the lightreflecting surface conforms to a predetermined shape; applying a resinto a surface of the glass member opposite the light reflecting surface;applying a reinforcing member and at lest one support structure to atleast one of the surface of the glass member opposite the lightreflecting surface and the resin; and curing the resin such that theresin, the reinforcing member and the support structure cooperate toform a rigid interlayer that supports the glass member and facilitatesmounting of the mirror assembly; wherein the resin shrinks as it curesand applies a compressive force to the surface of the glass memberopposite the light reflecting surface, the compressive force having amagnitude such that the entire cross-sectional thickness of the glassmember is maintained in a state of compression.